The overall goal of this project is to characterize the epidemiology of HIV and HCV among injecting drug users (IDUs) in Tijuana, Mexico. Recent data suggest a dynamic HIV sub-epidemic among IDUs on the northwestern Mexico-US border where injection drug use is now a major public health problem. The aims of this proposal are: 1) To determine HIV, HCV and syphilis prevalence and estimate HIV incidence based on the detuned assay among IDUs in Tijuana;2) To determine the extent to. which potentially modifiable individual, social and environmental factors influence the following outcomes among IDUs in Tijuana: i) prevalent HIV and HCV and incident HIV infections;ii) risk behaviors for blood borne infections (e.g., direct and indirect syringe sharing, shooting gallery attendance);iii) barriers to access and use of sterile syringes and injection paraphernalia within the context of legal syringe access through pharmacies. Individual level factors of interest include knowledge of HIV and HCV serostatus, recent onset injection and use of specific injection drug combinations (e.g., heroin and methamphetamine) and formulations (e.g., heroin tar versus powder);Social factors include emigration and personal network characteristics;Environmental factors include geospatial factors (e.g., distance from the border, frequency of border crossings, density of shooting galleries, pharmacies and drug treatment programs), and experiences with law enforcement. 3) To examine the social acceptability of interventions to reduce drug-related harm among the following stakeholders in Tijuana. To meet Aims 1 and 2, 2000 IDUs who have injected at least once in the prior 6 months will be recruited using respondent driven sampling, which will facilitate an unbiased estimate of HIV and HCV prevalence and associated risk behaviors. Face-to-face interviews will collect sociodemographic and behavioral data. To assess geospatial factors, we will use hand-held global positioning system devices to map cross-street locations of shooting galleries, pharmacies and drug treatment programs. To meet Aim 3,we will conduct in-depth interviews with three types of key informants (e.g., systems, interactor and street-level key informants). Although Mexico is considered a country of low HIV/AIDS prevalence with a concentrated epidemic, the window of opportunity for prevention is believed to be closing rapidly since IDU-associated HIV epidemics are often explosive and can quickly become generalized. This study will inform the development of culturally-appropriate interventions in one of the largest Mexican-U.S. cities, which has implications for policy making and program planning for both countries.